Can spreading iron in oceans reduce a lot of carbon dioxide in the atmosphere? Not as much or quickly as you'd think, said researchers at the Lawrence Berkeley National Laboratory.

After analyzing data from ocean-diving devices that trawled for carbon dioxide in the deep sea for over a year, Jim Bishop and Todd Wood concluded that phytoplankton aren't the carbon-dioxide removal machines that some believe them to be.

"Just adding iron to the ocean hasn't been demonstrated as a good plan for storing atmospheric carbon," says Bishop, who also is an earth sciences professor at UC Berkeley, in a statement. "What counts is the carbon that reaches the deep sea, and a lot of the carbon tied up in plankton blooms appears not to sink very fast or very far."

An upcoming issue of the journal Global Biogeochemical Cycles will publish the researchers' results, the lab said Wednesday.

There has been a long-held theory, called Iron Hypothesis, that stimulating plankton blooms could significantly remove carbon dioxide, a key greenhouse gas that contributes to global warming, from the atmosphere and deposit them in the ocean. Like plants on land, plankton would eat up carbon dioxide as part of its energy production process under the sun. There are two ways for carbon to sink and get buried in the deep sea. One is through the waste created by sea creatures that eat plankton. And the other is through the death of the plankton themselves. 

The earth's eco-system has its own, natural ways of producing and removing carbon dioxide from the atmosphere. But the system lacks the power to absorb the growing amount of carbon dioxide and other greenhouse gases produced by human activities such as generating electricity and driving cars.

Iron proved to be the steroid that could boost plankton growth, according to research by John Martin at the Moss Landing Marine Laboratories. Martin then proposed that seeding iron in the ocean could turn plankton into weapons for fighting global climate change.

Companies have emerged to try to make money from this carbon-sequestration idea. One of them, Planktos, gave up on the idea last year (see Planktos Seeks New Business Ideas). Climos, based in San Francisco, is still in business and is raising money and building a science team for a demonstration project, said CEO Dan Whaley.

Whaley said the Berkeley Lab researchers’ paper didn’t provide the necessary details, such as what types of plankton were observed, to show that phytoplankton aren’t good at burying carbon dioxide in the ocean.

“I have the utmost respect for Jim Bishop. But this is clearly not an iron experiment,” Whaley said. “He seems to suggest that growing more plankton won’t store much more carbon – but I think that the geological record argues that it has.”

Bishop and Wood said their research showed that the notion of global warming being halted or even reversed by boosting plankton growth is not as easily achieved as some have claimed.

The researchers based their findings on data provided by devices called Carbon Explorers, which were first launched in 2002 as part of project by the Moss Landing Marine Lab and the Monterey Bay Aquarium Research Institute. The project set out to test Iron Hypothesis in the ocean between New Zealand and Antarctica during the summer.

The Carbon Explorers not only collected data during the iron-seeding experiments, they also did so in the fall and winter for over a year, after evidence of the iron scattering had disappeared, the lab said. The devices were lowered into a depth of 800 meters and more.

Results from initial studies showed that, indeed, an artificially induced plankton bloom could remove 10 percent to 20 percent of the carbon from the ocean surface and deposit them to below 100 meters. The research team published a paper in the journal Science in April 2004.

Those results were based on data collected 60 days after the iron seeding. But data from the Carbon Explorers in the subsequent 16 months demonstrated that how much carbon can be sequestered by plankton blooms depended largely on the feeding and lifecycle of the zooplankton that eat phytoplankton.

Zooplankton don't get easy access to phytoplankton throughout the year, however, because of seasonable weather patterns and how they've adopted to surviving in darkness when sun doesn't shine in Antarctica during winter. As a result, the amount of carbon that can be absorbed by plankton isn't as great as anticipated, the Berkeley Lab researchers said.